Re: [PATCH 15/17] x86, mpx: do 32-bit-only cmpxchg for 32-bit apps

[Dave Hansen <dave@sr71.net>]

On 03/28/2015 01:39 AM, Borislav Petkov wrote:
> -	if (is_64bit_mm(mm)) {
> -		ret = user_atomic_cmpxchg_inatomic(actual_old_val_ptr,
> -				bd_entry_addr, expected_old_val, new_bd_entry);
> -	} else {
> -		u32 uninitialized_var(actual_old_val_32);
> -		u32 expected_old_val_32 = expected_old_val;
> -		u32 new_bd_entry_32 = new_bd_entry;
> -		u32 __user *bd_entry_32 = (u32 __user *)bd_entry_addr;
> -		ret = user_atomic_cmpxchg_inatomic(&actual_old_val_32,
> -				bd_entry_32, expected_old_val_32,
> -				new_bd_entry_32);
> -		if (!ret)
> -			*actual_old_val_ptr = actual_old_val_32;
> -	}
> -	return ret;
> +	if (is_64bit_mm(mm))
> +		return user_atomic_cmpxchg_inatomic(actual_old_val_ptr,
> +						   bd_entry_addr,
> +						   expected_old_val,
> +						   new_bd_entry);
> +	else
> +		return futex_atomic_cmpxchg_inatomic((u32 *)actual_old_val_ptr,
> +						    (u32 __user *)bd_entry_addr,
> +						    expected_old_val,
> +						    new_bd_entry);
>  }

That does look tempting, and I appreciate the analysis.

But, I'd really rather not hide this behind another layer of abstraction
in order to save a few variable declarations.  It's definitely _smaller_
code, but it's a little less obvious what is going on.